1. Chapter 12 Introduction to Atoms Christina Burk

2. Section One Development of the Atomic Theory

3. Atoms An atom is the smallest particle into which an element can be divided and still be the same substance. Atoms --> elements --> compounds A theory is a unifying explanation for a broad range of hypotheses and observations that have been supported by testing.

4. Democritus Proposes the Atom 440 B.C. Atoms are “uncuttable”. Atoms are small, hard particles made of a single material formed into different shapes and sizes. Atoms are always moving and they form different materials by joining together.

5. Aristotle 384-322 B.C. You can keep cutting an object in half over and over and never end up with an indivisible particle. He was later proven incorrect, but he had such a strong influence on popular belief that Democritus's ideas were largely ignored for centuries.

6. Dalton Creates an Atomic Theory Based on Experiments 1803 Elements combine in specific proportions to form compounds-ratios based on mass of elements. All substances are made of atoms. Atoms are small particles that cannot be created, divided, or destroyed. Atoms of the same element are exactly alike, and atoms of different elements are different. Atoms join with other atoms to make new substances.

7. Thomson Finds Electrons in the Atom 1897 Atoms are made of smaller particles called electrons. Electrons are negatively charged particles. Thomson experimented with a cathode-ray tube. Plum-pudding (chocolate chip ice cream) model-representation of an object or system.

8. Rutherford Opens an Atomic “Shooting Gallery” 1909 Atoms are made up of mostly empty space. In the center of the atom is a tiny, extremely dense, positively charged region called the nucleus. Negative electrons move in the empty space.

9. Bohr States That Electrons Can Jump Between Levels 1913 Electrons travel around the nucleus in definite paths. These paths are located in levels at certain distances from the nucleus. Electrons are located in levels a fixed distance from the nucleus and each other. Electrons can jump from a path in one level to a path in another level.

10. The Modern Theory: Electron Clouds Surround the Nucleus Electrons do not travel in definite paths. There are regions inside the atom where electrons are likely to be found- these regions are called electron clouds. Electrons are in a certain area, but they do not follow a certain path.

11. Section Two The Atom

12. How Small is an Atom? Aluminum atoms, which are average- sized atoms, have a diameter of about 0.00000003 cm. Atoms are extremely small!

13. What’s Inside an Atom? Even though an atom is very small, it contains even smaller particles in it- protons, neutrons, and electrons. Protons and neutrons make up the nucleus, which is the center of the atom. Electrons are found outside the nucleus.

14. Protons Protons are the positively charged particles of the nucleus. All protons are identical, and each proton has a positive charge. Because the masses of particles in atoms are so small, scientists developed a unit of measurement for them. The SI unit used to measure the masses of particles in atoms is the atomic mass unit (amu). Protons are 1 amu.

15. Neutrons Neutrons are the particles of the nucleus that have no charge. All neutrons are identical. Neutrons are slightly more massive than protons, but the difference in mass is so small that neutrons are also given a mass of 1 amu.

16. The Nucleus The nucleus is the small, dense, positively charged center of the atom. It contains most of the atom’s mass. Protons and neutrons are the most massive particles in an atom, yet the nucleus they form has a very small volume.

17. Outside of the Nucleus Electrons are the negatively charged particles in atoms. They are found moving around the nucleus within electron clouds. The charges of protons and electrons are opposite but equal in size (the atom is neutral). If the number of electrons is different from the number of protons, the atom becomes a charged particle called an ion. The mass of an electron is so small that it is usually considered to be zero.

18. How Do Atoms of Different Elements Differ? Protons and electrons are found in all atoms, and the simplest atom consists of just one of each. When there is more than one proton in an atom, a neutron must be added to hold the protons together, otherwise, since the protons are of the same charge, they would repel each other.

19. The Number of Protons Determines the Element The number of protons in the nucleus of an atom is the atomic number of that atom. Each element is composed of atoms that all have the same atomic number.

20. Are All Atoms of an Element the Same? Isotopes are atoms that have the same number of protons but have different numbers of neutrons. An unstable atom is an atom whose nucleus can change in composition. This type of isotope is radioactive. Isotopes of an element share most of the same chemical and physical properties.

21. How Can You Tell One Isotope from Another? You can identify each isotope of an element by its mass number. The mass number is the sum of the protons and neutrons in an atom. Electrons are not included in an atom’s mass number because their mass is so small that they have very little effect on the atom’s total mass. To find the number of neutrons in an isotope, subtract the atomic number from the mass number.

22. How Do You Calculate the Mass of an Element? Atomic Mass is the weighted average of the masses of all the naturally occurring isotopes of an element. To find the atomic mass, you multiply the mass number of each isotope by its percentage abundance in decimal form. Then add these amounts together.

23. What Forces Are at Work in Atoms? Gravity is the act between all objects all the time. The amount of gravity between objects depends on their masses and the distance between them. The Electromagnetic Force is when objects that have the same charge repel each other, while objects with opposite charge attract each other.

24. Forces At Work in Atoms The Strong Force is when protons push away from one another because of the electromagnetic force. A nucleus containing two or more protons would fly apart if it were not for the strong force. The strong force is greater than the electromagnetic force, so the nucleus stays together. The Weak Force is an important force in radioactive atoms. In certain unstable atoms, a neutron can change into a proton and an electron. The weak force plays a key role in this change.